When a frequency converter or a similar device is used for controlling the motion of a motor, it is desirable for the motor parameters be known. The machine parameters are used in control algorithms of the converter for accurate control of the load. The parameters of the machine can be identified using an identification procedure during the commissioning of the drive. In this identification run the converter performs one or more tests on the machine and estimates the parameters to be used for control.
In some situations, the load connected to the machine causes some restrictions to the drive so that the identification procedure cannot be carried out. The load connected to the controlled motor produces counter torque, which disturbs the identification run to the extent that the identification procedure does not provide accurate results that would enable precise control of the motor. It might also be possible that the identification run cannot be performed with the load at all due to maximum speed and/or maximum torque limits set for the load. In these situations, the identification run would involve completely taken off the controlled machine from the load. This is often cumbersome and sometimes even impossible if the machine to be controlled is an integral part of the mechanics of the load.
For these purposes, frequency converters may have an option for a stand-still identification procedure in which the rotor of the machine is not rotated. However, the stand-still procedures are able to identify only some of the parameters of the motor while other parameters are calculated using rated values of the machine.
FIG. 1 shows an L-equivalent circuit of an induction machine. The stator resistance Rs of the equivalent circuit can be quite easily identified using DC magnetization current, i.e. stator current is, produced with a constant stator voltage us. Rotor resistance RR and stray inductance σLs can be identified using a DC current with summed pulsed current or with some other injected current.
A disadvantage of the known stand-still identification methods is that the measurement of main inductance LM and the rotor time constant τr (=LM/RR) depending thereon is very tricky because the machine cannot be rotated. This is due to the fact that in stand-still methods, the used stator current pulses are almost entirely summed with opposing phases to the rotor current. This means that the corresponding changes in the magnetizing current im are relatively small, and thus the response from the main inductance to the test pulses is poor.
The known stand-still methods cannot achieve satisfying estimates for the main inductance and rotor time constant, and these parameters are usually approximated from slip frequency and power factor calculated from the rated values of the machine or from the cos θ value given as the rated value. On the other hand, the rated values are not necessarily exact so that the values obtained for the main inductance and rotor time constant with the known stand-still identification methods are inaccurate, which is reflected as a poorer performance of the control when compared to the performance achieved with parameters obtained in identification runs performed with rotating machines.
The accuracy of the voltage measurement should increase considerably so that sufficiently accurate estimates for the main inductance LM or rotor time constant τr could be calculated in stand-still methods. The increase in voltage accuracy is hard to achieve since, due to reduced costs, the output voltage in frequency converters is typically calculated using a measured DC bus voltage and output switch combination. In this kind of measurement, commutation delays and threshold voltages cause inaccuracies to the voltage measurement. These inaccuracies can be quite considerable when compared with the voltage response from the main inductance at the injection frequency.